Fuel cell plates with skewed process channels for uniform distribution of stack compression load

ABSTRACT

An electrochemical fuel cell includes an anode electrode, a cathode electrode, an electrolyte matrix sandwiched between electrodes, and a pair of plates above and below the electrodes. The plate above the electrodes has a lower surface with a first group of process gas flow channels formed thereon and the plate below the electrodes has an upper surface with a second group of process gas flow channels formed thereon. The channels of each group extend generally parallel to one another. The improvement comprises the process gas flow channels on the lower surface of the plate above the anode electrode and the process gas flow channels on the upper surface of the plate below the cathode electrode being skewed in opposite directions such that contact areas of the surfaces of the plates through the electrodes are formed in crisscross arrangements. Also, the plates have at least one groove in areas of the surfaces thereof where the channels are absent for holding process gas and increasing electrochemical activity of the fuel cell. The groove in each plate surface intersects with the process channels therein. Also, the opposite surfaces of a bipolar plate for a fuel cell contain first and second arrangements of process gas flow channels in the respective surfaces which are skewed the same amount in opposite directions relative to the longitudinal centerline of the plate.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is hereby made to the following copending applications dealingwith related subject matter and assigned to the assignee of the presentinvention:

1. "Apparatus For Supplying Electrolyte To Fuel Cell Stacks" bySpurrier, assigned U.S. Ser. No. 718,773 and filed Mar. 1, 1985.

2. "Fuel Cell Plates With Improved Arrangement Of Process Channels ForEnhanced Pressure Drop Across The Plates" by F. R. Spurrier et al,assigned U.S. Ser. No. 804,414 and filed Dec. 4, 1985 now U.S. Pat. No.4,631,239.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to fuel cells which convert thelatent chemical energy of a fuel into electricity directly throughelectrochemical reactions and, more particularly, is concerned with fuelcell plates employing skewed process channels for providing a moreuniform distribution of the compression load imposed on the fuel cellsin a stack thereof.

2. Description of the Prior Art

One common fuel cell system includes a plurality of subassemblies which,except for the top and bottom subassemblies, each include two bipolarplates between which is supported two gas electrodes, one an anode andthe other a cathode, and a matrix with an ion-conductive electrolyte,such as phosphoric acid, between the anode and cathode electrodes. Thesubassemblies, herein referred to as fuel cells, are oriented one atopanother and electrically connected in series (alternate electron and ionpaths) to form a fuel cell stack. The top end plate of the topsubassembly and the bottom end plate of the bottom subassembly are eachhalf-bipolar plates. Representative examples of such fuel cell systemare disclosed in U.S. Pat. Nos. to Kothmann et al (4,276,355;4,342,816), Kothmann (4,292,379; 4,324,844; 4,383,009) and Pollack(4,366,211) which, with the exception of U.S. Pat. Nos. 4,342,816 and4,383,009, are assigned to the assignee of the present invention.

Process gases, such as a fuel and an oxidant are supplied respectivelyto the anode and cathode electrodes via manifolds attached to the stackand channels defined in the bipolar plates. The fuel in the form ofhydrogen atoms when supplied to the anode dissociates into hydrogen ionsand electrons. The electrons are transmitted from the anode electrodeacross the bipolar plate to the next cell's cathode electrode, while thehydrogen ions migrate directly through the acidic electrolyte to itscell's cathode, where they react with electrons from another anode andoxygen to form water. This is repeated through the stack out through theends where the electron transfers from the last anode to the lastcathode as the other end of the stack is through the external circuitwhere useful work is produced.

The bipolar plates of the fuel cells basically function to structurallysupport and physically separate the individual fuel cells and providecavities for the process gases to access the anode and cathodeelectrodes where the electrochemical reaction occurs. The plates arenormally rectangular or square in shape with a series of generallyparallel channels formed in both top and bottom surfaces thereof.Recently, several problem areas have been recognized with respect to theprocess gas flow channel configurations found in conventional fuel cellbipolar plates.

First, a problem exists with respect to inadequate process gas flowdistribution to the fuel cells in the lower portion of the fuel cellstack as compared to in the upper portion. Conventional process channelsin bipolar plates tend to be relatively short and have few directionalchanges. As a consequence, process gas pressure drop in the horizontaldirection in traversing the plate channels within a given fuel cell issmall in comparison with the pressure drop in the stack distributionmanifold and piping system in the vertical direction. This results in aninherently poor flow distribution of the process gases to the fuelcells, more flow to the cells in the upper portion than in the lowerportion of the fuel cell stack. One approach to reduce this probleminvolves restricting the flow of process gases between the supply pipingand the manifold cavities adjacent to the stacks to artificiallyincrease the pressure drop in the parallel fuel cell circuits betweenthe supply and exit piping and thereby improve flow distribution.However, this approach complicates the stack design, increasing thenumber of parts and, thus, cost.

Second, a problem exists with respect to non-uniform distribution of acompressive load carried across the fuel cells which maintains the fuelcells operatively assembled together in the stack. Depending upon theparticular desired configuration for process gas flow through the fuelcell (i.e., crossflow, countercurrent, concurrent, or some combinationthereof), the flow channels in one surface of a bipolar plate may runeither perpendicular or parallel to the flow channels in the othersurface of the plate. Thus, with respect to any given fuel cell, theflow channels in the lower surface of the top plate will run eitherperpendicular or parallel to the flow channels in the upper surface ofthe bottom plate. Also, a typical fuel cell includes a sealing componentor gasket at the outer edges of the cell that fits between the platesand forms a boundary to separate the process gases from each other andfrom the external environment.

The areas where the lands or ribs defining the channels in the upper andlower plate surfaces overlap represent the areas of contact (through theelectrodes and electrolyte matrix) between the cell plates through whichthe compressive load is transferred. In regions where the ribs crossperpendicular to each other, the contact area between the opposing platesurfaces remains nearly constant, varying only to the extent thatmanufacturing tolerances affect the width of the ribs. In regions wherethe ribs are parallel to each other, the contact area is also a functionof manufacturing tolerances affecting the width of the ribs; but inaddition, variations in channel to channel placement, molding shrinkage,edge machining and assembly alignments (plate to plate) also affectcontact area. The net result is that parallel rib contact area variesmuch more than perpendicular rib contact area.

The cell components are quite susceptible to damage and prematurefailure if the contact areas are not controlled within certain limits,to limit contact stress and related cell strain. Any significantthickness variations associated with the plates compound the problem byproviding local areas of higher or lower stress. A low stress limit mustbe maintained to insure minimum electrical contact resistance foruniform cell performance. To optimize uniform cell performance it isdesirable to have both parallel and perpendicular contact areas nearlythe same so the compression load is carried uniformly across the cell.

Consequently, a need still exists for improvements in the configurationof the fuel cell process gas flow channels in order to alleviate theproblems of inadequate flow distribution across the fuel cells in astack thereof and non-uniform compression loading of the fuel cells.

SUMMARY OF THE INVENTION

The preferred embodiments of the fuel cell plates, as disclosed herein,include several improved features which meet the aforementioned needs.While the improved features are particularly adapted for workingtogether to facilitate improved efficiency and reliability of fuel cellsincorporated in a common stack, it is readily apparent that suchfeatures may be incorporated either singly or together in various fuelcell configurations.

Some of the several improved features comprise the invention claimed inthe second copending application cross-referenced above; however, all ofthe improved features are illustrated and described herein forfacilitating a complete and thorough understanding of those of thefeatures comprising the present invention.

The present invention relates to an improved skewed arrangement ofprocess gas channel configurations on opposite plate surfaces of abipolar plate.

Accordingly, the present invention is directed to an improvement in anelectrochemical fuel cell including an anode electrode, a cathodeelectrode, an electrolyte matrix sandwiched between the electrodes and apair of plates above and below the electrodes. The plate above theelectrodes has a lower surface with a first group of process gas flowchannels formed thereon, whereas the plate below the electrodes has anupper surface with a second group of process gas flow channels formedthereon. The channels of each group extend generally parallel to oneanother. The improvement is comprised by the process gas flow channelson the lower surface of the plate above the anode electrode and theprocess gas flow channels on the upper surface of the plate below thecathode electrode being skewed in opposite directions such that contactareas of the surfaces of the plates through the electrodes are formed incrisscross arrangements. In addition, at least one groove is provided inareas of the surfaces of the plates where the channels are absent (butpresent in the plate on the opposite side of the cell) for holdingprocess gas and increasing electrochemical activity of the fuel cell.The groove in each plate surface does not have to intersect with theprocess channels therein, but preferably, the groove does intersect andis in flow communication with the process channels on the same surfaceof the plate.

Also, the present invention is directed to a bipolar plate for a fuelcell, comprising: (a) a base having opposite surfaces and a longitudinalcenterline; (b) a first arrangement of process gas flow channels on oneof the surfaces of the base; and (c) a second arrangement of process gasflow channels on the other of the surfaces of the base. The firstarrangement of channels are skewed the same amount in one direction asthe second arrangement of channels are skewed in the opposite directionrelative to the longitudinal centerline of the base. The bipolar platealso includes a first arrangement of ribs on the one opposite surface ofthe base which define the first arrangement of gas flow channels, and asecond arrangement of ribs on the other opposite surface of the basewhich define the second arrangement of gas flow channels. Each of therib arrangements define grooves in areas of the surfaces where thechannels are absent for holding process gas and increasingelectrochemical activity of the fuel cell. Preferably, the groovesintersect with the channels on the same surface of the base.

These and other advantages and attainments of the present invention willbecome apparent to those skilled in the art upon a reading of thefollowing detailed description when taken in conjunction with thedrawings wherein there is shown and described an illustrative embodimentof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the following detailed description, reference will bemade to the attached drawings in which:

FIG. 1 is an elevational view, partly in section, of a fuel cell stackmodule, with portions omitted for purposes of clarity, whichincorporates fuel cells constructed in accordance with the presentinvention.

FIG. 2 is a vertical cross-sectional view of a single fuel cell removedfrom the fuel cell stack module of FIG. 1, with electrolyte fill holesand flow grooves and process gas channels and manifolds associatedtherewith being omitted for purposes of clarity.

FIG. 3 is a diagrammatic representation of a bipolar plate surfacehaving process gas flow channels defined in a serpentine configurationwith the channels being represented by the lines, the channels alsobeing skewed relative to the sides of the plate in accordance with oneof the improved features of the present invention.

FIG. 4 is an enlarged fragmentary sectional view taken along line 4--4of FIG. 3, showing the adjacent ribs and channels of the plate.

FIG. 5 is a diagrammatic representation of a bipolar plate surfacehaving process gas flow channels defined in a modified serpentineconfiguration constituting one of the improved features of the inventionclaimed in the second cross-referenced application with the lands orribs defining the channels therebetween being represented by the linesand the interruptions in the lines representing slots through the ribswhich interconnect the channels in flow communication, the channels inthe modified configuration also being skewed relative to the sides ofthe plate in accordance with the present invention.

FIG. 6 is an enlarged fragmentary sectional view taken along line 6--6of FIG. 5, showing a channel and a slot defined in a rib of the plate.

FIG. 7 is an enlarged fragmentary sectional view taken along line 7--7of FIG. 5, showing channels, ribs and a slot defined in one rib of theplate.

FIG. 8 is a diagrammatic representation of a bipolar plate having afirst plurality of process gas flow channels, being shown in solid lineform, defined in a top surface of the plate in a first serpentineconfiguration skewed relative to a longitudinal centerline of the platein a counterclockwise direction about the center of the plate and asecond plurality of process gas flow channels, being shown in dashedline form, defined in an opposite bottom surface of the plate in asecond serpentine configuration skewed relative to the longitudinalcenterline of the plate in a clockwise direction about the center of theplate and in an opposite sense to the first plurality of channels.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views. Also, in thefollowing description, it is to be understood that such terms as"forward", "rearward", "left", "right", "upwardly", "downwardly", andthe like, are words of convenience and are not to be construed aslimiting terms.

In General

Referring now to the drawings, and particularly to FIG. 1, there isshown an electrochemical fuel cell stack module, generally designated bythe numeral 10, which includes a plurality of fuel cell stacks 12. Eachfuel cell stack 12 contains a multiplicity of repeating fuel cells 14(one of which is diagrammatically illustrated in FIG. 2) whichincorporate the features of the present invention as well as those ofthe cross-referenced application.

For maintaining the fuel cell stacks 12 in an optimum electrolyte-wettedcondition, the fuel cell stack module 10 also includes an electrolytereservoir 16, a distribution block 18, and a pump 20 for supplyingelectrolyte from the reservoir 16 via primary supply line 22 to thedistribution block 18. The fuel cell stacks 12 are connectedindividually in flow communication with the electrolyte distributionblock 18 and reservoir 16 by respective stack supply lines 24 and returnor drain lines 26. Specifically, the distribution block 18 includes anelectrolyte chamber (not shown) for each stack 12 in the module 10 and asingle overflow chamber (not shown). The stack supply lines 24interconnect electrolyte fill holes (not shown) in the top of therespective stacks 12 with the distribution block electrolyte chambers,whereas an overflow return line 28 connects the electrolyte overflowchamber of the distribution block 18 with the reservoir 16. For a moredetailed description of this electrolyte supply system associated withthe module 10, attention is directed to the first applicationcross-referenced above, the disclosure of which is incorporated hereinby reference.

In operation, a predetermined electrolyte volume is forced by the pump20 through the primary supply line 22 to the distribution block 18.Within the block 18, an equal amount of electrolyte is delivered to eachelectrolyte chamber. Excess electrolyte flows into the overflow chamberand is returned to the reservoir 16 through the overflow return line 28.Electrolyte from the chambers is delivered through the stack supplylines 24 to the fill holes in the tops of the respective stacks 12.

In order to avoid creation of a short circuit through theelectrically-conductive electrolyte, and damage to the fuel cell stacks12, electrolyte is delivered in predetermined periodic pulses of shortduration by the pump 20 rather than by continuous flow. Gravitationalforce assists circulation of electrolyte through the fill holes andgrooves (not shown) of the fuel cell stacks. Any electrolyte notabsorbed passes out of the stacks 12 through the drain holes (not shown)and is returned to the reservoir 16 through the drain lines 26.

As mentioned, each fuel cell stack 12 of the module 10 includes amultiplicity of repeating fuel cells 14 (only one being shown) beingarranged such that each cell is electrically connected in series withone another in a conventional manner (not shown). As seen in FIG. 2,each fuel cell 14 basically includes top and bottom bipolar plates 30between which are sandwiched an anode electrode 32, anelectrolyte-containing porous matrix 34 and a cathode electrode 36 (thisarrangement may be inverted). Also, shims or gaskets 38 are ordinarilyprovided for sealing about the peripheries of the electrodes. In anexemplary embodiment, each bi-polar plate 30 is composed of a relativelythick rigid material such as compression molded graphite resincomposite, while each electrode 32,36 is a thin sheet of a porousgraphite material provided with a porous graphite fiber backing foradded structural integrity. The matrix 34 is composed of thintissue-like sheets made of porous graphite wetted or saturated with anelectrolytic acid, such as concentrated phosphoric acid, through fillholes and flow grooves 40 (not seen in FIG. 2, but in FIG. 3) in theplate 30 which communicate with the supply and drain lines 24,26. Manyother materials and structures can also be used to compose thecomponents of the fuel cell 14.

Preferably, hundreds of the repeating fuel cells 14 (again, only one isshown) are united to form the fuel cell stack 12. Thus, the top bipolarplate 30 of each fuel cell also acts as the bottom bipolar plate 30 forthe fuel cell immediately above it and the bottom bipolar plate 30 ofeach fuel cell also acts as the top bipolar plate for the fuel cellimmediately below it. Also, a full fuel cell stack typically includesend plates (not shown) in the form of half-bipolar plates, with a tophalf-bipolar plate serving as the upper end plate and a bottomhalf-bipolar plate serving as the lower end plate.

The bipolar plates 30 are typically provided on opposite sides with aset of process channels (not seen in FIG. 2), including fuel channels onone side and oxidant channels on the other side, being configured inaccordance with the improved features of the present invention, as willbe described below. A fuel, such as hydrogen, flows through the fuelprocess channels, whereas an oxidant, such as a halogen, air or otheroxygen-containing material, flows through the oxidant process channels.Fuel inlet and outlet manifolds (not shown) and oxidant inlet and outletmanifolds (not shown) are typically attached to respective inlet andoutlet regions of the fuel cell stacks 12 in communication with the fueland oxidant channels to provide fuel and oxidant flows to and from thestack. Electrical power and heat are generated by the interaction of thefuel and oxidant through the electrodes 32,36 and electrolyte matrix 34.An exemplary fuel cell 14 utilizes hydrogen fuel, air as the oxidant andphosphoric acid as the electrolyte.

Heat is generated by the electrochemical reaction and, accordingly, eachof the stacks 12 ordinarily includes cooling modules (not shown).Dependent upon the operating temperatures desired, the cooling modulesare placed between the fuel cells 14 at selected positions within thestack 12. A cooling module may, for example, be placed betweenapproximately every fifth cell to every eighth cell. Each module ispreferably comprised of a material similar to that of the bipolar plates30 and has air cooling passages therethrough.

Improved Fuel Cell Plate Process Channel Arrangement and Configuration

Referring now to FIGS. 3 and 5 and to the features constituting theinvention of the second cross-referenced application, there is seen thetwo opposite surfaces 48 (FIG. 3) and 50 (FIG. 5) on the base structure52 of a bipolar plate 30 containing two different arrangements ofprocess gas flow channels. (The fact that the channel arrangements areskewed relates to the present invention to be described later inreference to FIG. 8). In FIG. 3, one surface 48 of the plate 30 isprovided with an arrangement of ribs 54 (see also FIG. 4) which definetherebetween an arrangement of gas flow channels 56 having a generallyserpentine configuration. The channels 56 (being represented by thelines in FIG. 3 whereas the ribs 54 are the remaining space between thelines) make a number of turns which increases the tortuousness of theprocess gas paths therethrough across the plate 30. In such manner, thepressure drop in the horizontal direction in the fuel cell issubstantially increased (in relation to that in the vertical directionthrough the stack supply and exit passages), thereby improving theplate-to-plate flow distribution.

In FIG. 5, the other surface 50 of the plate 30 is provided with anotherarrangement of ribs 58 (see also FIGS. 6 and 7) which definetherebetween another arrangement of gas flow channels 60 having agenerally modified serpentine configuration. In this configuration, theribs 58 have formed therein a multiplicity of spaced slots 62, as seenalso in FIGS. 6 and 7. (In FIG. 5, the ribs 58 are represented by thelines, the channels 60 by the spaces between the lines, and the slots 62by the interruptions along the lines.) The channels 60 are generallylinear and extend generally parallel to one another. The slots 62interconnect and provide flow communication between adjacent channels60. Adjacent pairs of the channels 60 are interconnected to one anotherin flow communication. As a general design principle, the slots 62 ineach one rib 58 are located in offset relation to the slots 62 in, theribs adjacent to the one rib, and the slots in any one rib 58 aregenerally aligned with the slots 62 in every other rib 58. Thus, whilethe channels 60 define generally longitudinal linear paths for gas flow,the spaced slots 62 facilitate cross channel flow of process gas in astaggered fashion which creates a series of generally parallelmini-serpentine flow paths extending transverse to the longitudinal gasflow along the channels 60.

More particularly, the ribs 58 and channels 60 can be divided into threebasic groups. The first group of ribs 58a have offset slots 62, and theslots 62 in one rib 58 are aligned with those in every other rib, asmentioned earlier. The ribs 58a of the first group also have anoutermost rib 58a' and an innermost rib 58a" as well as a succession ofribs 58a therebetween. The outermost rib 58a' has the lowest number ofthe slots 62 therein and displaced the furthest from the entry edge 64of the plate base 52. The innermost rib 58a" has the highest number ofthe slots 62 therein and displaced the closest to the entry edge 64 ofthe plate base 52. With respect to the slots 62 in the succession of theribs 58a between the outermost and innermost ones, their numbergenerally increases going from those of the ribs 58a closer to theoutermost rib 58a' to those of the ribs 58a closer to the innermost rib58a". The first group of gas flow channels 60a at ones of their oppositeends open at the gas entry edge 64 of the plate base 52. The channels60a are connected together in flow communication in adjacent pairs atthe others of their opposite ends.

The second group of ribs 58b, as mentioned before, have slots 62 locatedin offset relation and the slots in any one rib 58 are generally alignedwith the slots 62 in every other rib 58. The second group of gas flowchannels 60b are connected together in flow communication in adjacentpairs at their respective opposite ends.

The third group of ribs 58c and channels 60c are substantially identicalto the first group thereof when rotated 180 degrees, and thus need notbe described in detail. Suffice it to say that the ribs 58c have thesame arrangement of slots 62 as the ribs 58a, and have innermost andoutermost ribs 58c' and 58c" and the succession of ribs 58ctherebetween. The slots 62 are found in the same pattern in the thirdgroup as in the first group of ribs. The third group of gas flowchannels 60c at ones of their opposite ends open at the opposite exitedge 66 of the plate base 52 and are connected together in flowcommunication in adjacent pairs at their others of their opposite ends.

As is apparent in FIG. 5, the second group of ribs 58b and channels 60bare disposed between the first and third groups thereof with the slots62 in the innermost ribs 58a",58c" of the first and third groups thereofinterconnecting and providing flow communication between respectiveadjacent channels 60a,60b in the first and second groups thereof andrespective adjacent channels 60b,60c in the second and third groupsthereof.

The modified serpentine configuration in FIG. 5 of the channels 60 andribs 58 which contain the slots 62 providing cross flow between thechannels 60 provides an even more complex tortuousness of the processgas paths across the plate 30 and thereby improves pressure drop andplate-to-plate flow distribution even further. In addition, the wavyconfiguration or serpentine-like mini-paths (see dashed lines witharrows in FIG. 5) superimposed on the general channel direction avoidthe tendency to shear the electrodes which can arise under certainconditions of misalignment due to tolerance variations in molded platesusing counterflow channel arrangements.

Finally, in FIG. 8, another improved feature which constitutes thepresent invention relates to the skewed arrangement of the process gaschannels 68,70 on the opposite surfaces of the bipolar plate 72. Whilethe channel arrangements in FIG. 8 have identical serpentineconfigurations, they can be of any arrangement which has some generallyparallel channel portions (The serpentine and modified or enhancedserpentine channel arrangements of FIGS. 3 and 5 are also skewed). Whenthe two of the plates 72 are provided in a fuel cell, the process gaschannels on the lower surface of the one plate above the anode electrodeand the process gas channels on the upper surface of the other platebelow the cathode electrode will be skewed in opposite directions. Thus,with respect to each bipolar plate 72 used in assembly of the fuelcells, the process channels 68 (shown in solid line form in FIG. 8) onthe upper side of the plate 72 are skewed the same amount in onedirection as the channels 70 (shown in dashed line form in FIG. 8) areskewed in the opposite direction relative to a common longitudinalhorizontal centerline 74 of the plate. The skewed arrangement preventsgas channel misalignment prevalent in parallel channel arrangements dueto manufacturing and assembly tolerances, and provides a uniform cellcontact area resulting in a more uniform distribution of fuel cell stackcompression loads.

As stated above, the skewed arrangement of process gas channels isapplicable to any channel arrangement that has parallel channels. Theamount of skew angle is chosen large enough to ensure that channelalignment tolerances do not bring the channels back into parallelrelationship. Calculations using various skew angles for a given lengthplate, a specific rib width and groove pitch show that any skew angleselected for such a plate will produce a constant contact area tonominal area ratio (0.16). Therefore, the amount of skew angle should beno larger than required in order to minimize the loss in cell activearea (i.e., the area in which process gas channels, above and below theelectrodes, would intersect when overlaid on each other) for specificsize plate channels. The loss in cell active area can be substantiallyreduced with the addition of one or more gas-holding grooves 76 in theareas where the channels 68,70 on opposite sides of the fuel cell do notintersect. However, preferably the gas-holding grooves 76 intersect andare in flow communication with the process channels on the same side ofthe bipolar plate. The number of grooves 76 that may be added aredetermined by the specific size of the channeled plate and the skewangle used.

It is thought that the present invention and many of its attendantadvantages will be understood from the foregoing description and it willbe apparent that various changes may be made in the form, constructionand arrangement thereof without departing from the spirit and scope ofthe invention or sacrificing all of its material advantages, the formhereinbefore described being merely a preferred or exemplary embodimentthereof.

We claim:
 1. In an electrochemical fuel cell including an anodeelectrode, a cathode electrode, an electrolyte matrix sandwiched betweensaid electrodes and a pair of plates above and below said electrodes,said plates having corresponding pairs of opposite edges, said plateabove said electrodes having a lower surface with a first group ofprocess gas flow channels formed thereon and said plate below saidelectrodes having an upper surface with a second group of process gasflow channels formed thereon, said channels of each group extendinggenerally parallel to one another, the improvement which comprises:saidprocess gas flow channels on said lower surface of said plate above saidanode electrode and said process gas flow channels on said upper surfaceof said plate below said cathode electrode being open at their oppositeends at the same ones of said corresponding edges of said plates andbeing skewed substantially the same amount in opposite directions suchthat contact areas of said surfaces of said plates through saidelectrodes are formed in crisscross arrangements; and at least onegas-holding groove defined in areas of said surfaces of said plateswhere said channels are absent for holding process gas and increasingelectrochemical activity of said fuel cell.
 2. The fuel cell as recitedin claim 1, wherein said at least one gas-holding groove in each platesurface intersects with said process channels therein.
 3. A bipolarplate for a fuel cell, comprising:(a) a base having a pair of oppositesurfaces with pairs of opposite edges and a longitudinal centerline; (b)a first arrangement of process gas flow channels on one of said surfacesof said base; (c) a second arrangement of process gas flow channels onthe other of said surfaces of said base; (d) said first arrangement ofchannels being skewed the same amount in one direction as said secondarrangement of channels are skewed in the opposite direction relative tosaid longitudinal centerline of said base; (e) said first and secondarrangements of channels on said opposite surfaces of said base beingopen at their opposite ends at the same corresponding edges of saidsurfaces of said base; (f) at least one gas-holding groove in areas ofsaid surface of said plates where said channels are absent for holdingprocess gas and increasing electrochemical activity of said fuel cells.4. The bipolar plate as recited in claim 3, further comprising:a firstarrangement of ribs on said one of said opposite surfaces of said basedefining said first arrangement of gas flow channels; and a secondarrangement of ribs on said other of said opposite surfaces of said basedefining said second arrangement of gas flow channels; each of said ribarrangements also defining a plurality of said gas-holding grooves inareas of said surfaces where said channels are absent for holdingprocess gas and increasing electrochemical activity of said fuel cell.5. The bipolar plate as recited in claim 3, wherein said at least onegas-holding groove in each plate surface intersects with said processchannels therein.